1. Field of the Invention
The invention relates to a nuclear reactor provided with automatic safe shutdown capability. The invention particularly relates to a nuclear reactor provided with a plurality of hydraulically supported spherical bodies having a high neutron absorption cross section, which fall by gravity into the core region of the reactor when the flow of supporting fluid is shut off.
2. Prior Art
Nuclear reactors usually incorporate an emergency shutdown system, which introduces some form of neutron absorbing material into the core. The emergency shutdown system has a basic requirement that injection of the neutron absorbing material should be simple, and that the neutron absorber should remain in the core in the event of any credible failure in the reactor system. One such system utilized heretofore was provided by the normal reactor control rods. The control rods are introduced through the top of the reactor and are raised and lowered generally by mechanical means such as a motor which operates via a clutch, gears, or the like. In an emergency, a clutch is disengaged and the control rods are allowed to fall into the core to shut down the reactor.
Such a system has not been altogether satisfactory. Specifically, there is a possibility that a mechanical device such as a clutch could not be disengaged, or that some fault may have occured which would distort the passage through which the control rods have to pass, such that they would bind, preventing full insertion of the rods into the core, and it would not be possible to shut down the reactor. Thus, numerous other backup shutdown systems have been proposed.
U.S. Pat. No. 3,147,188 suggests a reactor shutdown apparatus using neutron absorber balls. The shutdown apparatus comprises a restraint means for releasably restraining a multitude of discrete bodies of materials which are magnetic and possess neutron absorbing properties. The restraint means comprises at least one pair of magnetic pole pieces of opposite polarity, the apparatus being operated to release the discrete bodies by demagnetization of the pole pieces, whereby said bodies are moved under the influence of gravity into the core of the reactor.
U.S. Pat. No. 3,228,847 suggests a reactor control system which includes a control assembly for controlling neutronic flux. The control assembly comprises an inner tube extending from a nonactive region of the reactor into the active region, and an outer tube surrounding the inner tube and spaced therefrom. The outer tube has a closed end and the inner tube has an open end adjacent and spaced from the closed end of the outer tube. Neutron absorbing particles are positioned between the inner and outer tube for movement along the tube under the force of flow. The neutron absorbing particles are moved out of the active region of the reactor by fluid flow and fall back into the active region under the influence of gravity when the flow is shut off.
U.S. Pat. No. 3,257,286 suggests a ball-type control for a nuclear reactor. A number of elongated conduits are positioned in the nuclear reactor so that the first section of the conduit is located within the core and an adjoining second section is located exteriorly of the core. Each conduit holds a number of individual bodies, each of which contains a high neutron absorption cross-section material. The movement of the neutron absorber bodies within the conduits is achieved by providing a source of pressurized fluid available to each end of the conduit for selectively positioning the neutron absorber bodies within the first and second sections of the conduit. It is stated that a fission reactor can be started up, shut down, or reactivity controlled during reactor operations by varying the location of the absorber bodies.
U.S. Pat. No. 3,347,747 discloses a control organization and method for a nuclear reactor. The reactor is provided with a number of laterally spaced vertical passageways in the region of the core and distributed throughout the area thereof. The passageways include a lower portion which extends generally throughout the height of the core and an upper portion which extends above the core into the reactor vessel. Positioned within and confined in each passageway is a movable means which contains a poison and which is movable from a lower position within the region of the core to an upper position in the passageway, where it is generally above the core. The poison-containing means is moved by gravity to its lower position and is moved from its lower to its upper position by means of a fluid which is directed upward in the passageway.
U.S. Pat. No. 3,682,771 relates to an emergency nuclear reactor shutdown system for a gas cooled graphite moderated reactor. The system comprises a U-shaped tube with one limb located within the core and the other located externally thereof. Inside the tube for movement therein is a first column of spheres of neutron absorbing material. Connected to the external limb of the tube is a reservoir for a second column of spheres of non-neutron absorbing material of a greater weight than the other spheres. The two columns are in endwise contact inside the tube. When the second column of spheres is released, its greater weight urges the first column into the reactor core and retains it there.
While all the foregoing suggested techniques appear to offer advantages over reliance solely on a control rod system, they are not without problems. More particularly, the present inventors have found that when using, for example, hydraulically supported neutron absorbing bodies, that excessively high fluid flows are required to move the bodies from a core zone into a nonreactive zone. Further, when such flow has been established, the pressure drop across the bodies is excessively high. Moreover, even when maintaining sufficient pressurized fluid to maintain the required high pressure differential across the neutron absorbing bodies, it still is difficult, if not impossible, to maintain all of the bodies reliably out of the active region of the core. Another disadvantage of several of the proposed methods is that frequently it is desired to operate the reactor at less than full power, whereas in the prior methods which utilize hydraulically supported bodies, the flow required to move the bodies out of the core region and that flow at which the bodies start to fall back into the core region are very near the same. Thus, any significant decrease in flow results in the bodies dropping back into the core region. Obviously, there still is need for an improved reactor which includes a plurality of hydraulically supported neutron absorbing balls to provide an inherently safe shutoff of the reactor in the event of loss of coolant flow.